AND GEOLOGY OP THE COTiUMBIA. 103 



they must be parted with the hands ; or among the tangled box-alders along river 

 margins, or the jungles of prickly " devil's-elubs " that rise as high as one's head in 

 swamps. Windfalls and snow-slides, where great trunks of spruce or Tkvja gigantea lie 

 heaped upon one another, also form most disheartening obstacles at times. On the 

 western slope, continued rains and swollen creeks add fretiuently to the difficulties of 

 travel. 



II. — Microscopic Petrography of Eock Specimens. 



(1.) Tlie Spillimichene Region. 



Slates.— The soft clayslates of the range between the Spillimichene and the 

 Columbia belong in all probability with the Palœozoic slates of the Rocky Mountains and 

 need not be described here. They readily crumble to clay and so are not easily made into 

 sections thin enough for microscopic work. The harder, somewhat more crystalline 

 slates south-east of the northern branch of the Spillimichene are more easily handled 

 and are more characteristic of the Selkirks. 



Macroscopic Description.— These slates are very fissile, the cleavage crossing at varying 

 angles the planes of stratification indicated by bands of lighter and darker grey. They 

 often contain cubes of pyrite, sometimes distorted by the pressure that formed the 

 cleavage. In a crystal measured, the angles between the faces meeting in edges exposed 

 to comj^ression were enlarged from 90° to 93° 30'. 



Fig. 2.— Distorted Crystals of Pyrite iu Slate. 



Microscopic. — Thin sections show a micro-crystalline or crypto-crystalline structure 

 with many opaque, dusty looking particles, especially in the darker bauds. Splinters of 

 the rock grow lighter in color when heated in a Bunsen burner, showing that at least 

 part of the dark substance is carbon. The transparent parts have not a clastic look. 

 Quartz may form a part of the obscurely anisotropic substances, but chlorite or some 

 related fibrous or scaly mineral seems more important. The vague fibres and oblong 

 sections seem to have extinction parallel to the chief sections of the Jiicols. A 

 scaly, slightly dichroic mineral, occurring in larger particles, is perhaps muscovite. Im- 

 mense numbers of minute rutile needles are scattered through the rock, often iu groups 

 with roughly radiating points, and sometimes with characteristic knee-shaped twins. '■ 

 Between crossed uicols the rutile needles gleam oiit as brilliantly-colored threads. Small 

 portions of a yellow brown, slightly translucent substance scattered here and there 

 may be hydrous sesquioxide of iron. 



' See plate xviii, fig. 1. 



